1. Field of the Invention
The present invention relates to a battery charger and, more particularly, to a battery charger which automatically compensates for the voltage drop across the battery terminals and any battery protection circuitry to optimize the voltage applied to the battery, making it suitable for charging various types of battery packs including Lithium Ion (LiIon) batteries which require both constant current and constant voltage charging.
Constant voltage battery charging as may be required by nickel-cadmium (NiCd), nickel metal hydride (NiMH), alkaline, as well as lithium ion batteries and the like, is provided in a battery charger with a regulated voltage source for constant voltage charging in which a first voltage measurement across the battery is taken while the battery is being charged, and a second voltage measurement across the battery is taken while the charging current is interrupted. A determination of the potential difference between the first voltage and the second voltage identifies a voltage drop across the terminals and/or protection circuitry of the battery pack, which is used for adjusting the setting of the voltage source to provide the optimum voltage level at the rechargeable cells of the battery pack. Automatic compensation of the voltage source thus provides battery cells under charge with the optimum constant voltage level for recharging, which is particularly well suited for charging lithium ion batteries.
2. Description of the Prior Art
Various portable devices and appliances, such as cellular phones, require rechargeable batteries. Various types of rechargeable batteries are known to be used in such applications. For example, nickel-cadmium (NiCd), nickel metal hydride (NiMH), as well as lithium ion batteries are known to be used. Because of the different charging characteristics of such batteries, different battery chargers are required. For example, both nickel-cadmium (NiCd), as well as nickel metal hydride (NiMH), require constant current charging. On the other hand, lithium ion batteries require constant current charging up to a certain voltage value and constant voltage charging thereafter. Because of the different charging requirements, different charging circuits are often required.
Standard battery packs normally consist of one or more battery cells disposed in a modular housing with external contacts for easy and convenient coupling with the portable device in which it is used. Smart battery packs, in addition to the battery cells, normally include a memory storage device which contains information regarding the characteristics of the battery as well as the battery type.
Some smart battery packs are known to include a microcontroller which allows communication by way of a bi-directional communication line with the battery charger regarding various battery characteristics. Examples of such smart battery packs are disclosed in: "Smart Battery Specifications", .COPYRGT.1993 Duracell Inc., Intel Corporation, hereby incorporated by reference. Because of the differences between the standard battery packs and the smart battery packs, different chargers are used for the smart battery packs and the standard battery packs.
Battery chargers for charging batteries which require constant current charging and batteries which require constant current and constant voltage charging, such as lithium batteries, are known in the art. Battery chargers are also known that are adapted to automatically sense the type of battery connected to the battery charger and provide the appropriate charging characteristic. As mentioned above, such battery chargers are used for various portable devices, such as cellular phones. Cellular phone battery chargers are commonly available as single pocket and dual pocket devices. Dual pocket devices are known to be used for charging a spare battery, as well as the battery connected to the cellular phone. Unfortunately, with known dual pocket battery chargers, each pocket is known to be treated independently. In particular, in situations in which batteries to be charged are disposed in both pockets, the battery in the active pocket is normally fully charged before any servicing of the battery in the other pocket is done. Lithium ion batteries are known to take 3-4 hours to charge. Should a second battery be placed in the inactive pocket while a lithium battery is being charged in an active pocket, the second battery could remain in the inactive pocket for 3-4 hours before charging is even commenced. If the second battery also happens to be a lithium battery, it could take from 6-8 hours for the second battery to be charged from the time the second battery is inserted in the inactive pocket. Unfortunately, the end user will normally not be aware of such a limitation in the charging system.
In known lithium ion battery chargers, as discussed above, a constant current and a constant voltage are used for charging. Known battery chargers maintain a pre-determined charging voltage at the charging terminals of the charger and adjust the current in a way that this voltage remains constant. However, between the charging terminals and the battery electrodes are two series impedances: a) the battery pack contact resistance with the charger terminals and b) the protection circuit between the battery pack terminals and the battery electrodes. When voltage measurements of the battery charger and battery pack terminals are compared to the battery cell electrodes, up to several hundred millivolts are known to be lost. This voltage loss results in a significant increase in the charge time of the battery. It would be desirable therefore to provide an improved method for charging lithium ion batteries which would be able to predict or dynamically measure the loss between the charging terminals and the battery electrodes. This loss term would then be added to the prescribed charging voltage of the particular lithium charging system under charge.